Rename SpdyHeaderBlock to Http2HeaderBlock. Salvaged from a botched Zwingli attempt: http://refactor/16032281396283088571 created by go/Zwingli Tested: TAP for global presubmit queue passed after automated deflaking of failures: http://mondo/deflaker/run/ea979a75-dd33-4574-acde-7191db6ea908 http://mondo/deflaker/run/51cec644-aef6-434d-bebd-6cff71dc319b http://test/OCL:338336292:BASE:338288991:1603328760683:25591553 PiperOrigin-RevId: 338474585 Change-Id: I3a6fb53e8e3a41386ac9eefc01fb5d32b158d044